SH-009 Monosynth is an emulated Roland SH-09(Roland does not endorse this plug-in of course). It is pretty close in functionality and sound to the hardware version. Highlights include a LFO modulator with random / sample-and-hold step voltage that can be used to modulate the oscillator, the filter, and the pulse width. It also includes an invert switch for the envelope, which can modulate the filter and pulse width, as well as the amplifier.

Ridiculously Red is intended for live use due to its drone capabilities with knobs that automate knobs, but it can also be used normally to create dirty & gritty leads, thick basses, and pads with a sense of movement.

Luno is (possibly) an almost exact emulation of a Juno 106. It can create great leads, basses & pads. The pads use quite a bit cpu because of the configuration, but can probably be tamed with delay fx insert, & by turning the envelope down.

Harmonical is an unusual synth based on spherical harmonics. Basically, instead of the usual saw/square wave, etc. oscillators, it uses a 3D sphere as a sound source, the vertices of which are modulated via spherical harmonics. It displays the sphere(s) in an OpenGL display.

Harmonical is available as a free VSTi plug-in for Windows and Mac OS X. The source code is also available.